Conventionally, in plasma CVD apparatus used in processes of producing semiconductors such as LSI, etc., large amounts of perfluoro compounds such as CF4, C2F6, SF6, NF3 etc. are used as a cleaning gas for CVD chambers in order to remove deposits on the surface of the inner wall or other surfaces of the CVD chambers during film forming treatment. These compounds are stable, and have a very long life in the atmosphere and a high infrared absorption. With regard to global warming potential (GWP100) of the above perfluoro compounds, on the basis of carbon dioxide being 1, CF4 has 6500, C2F6 has 9200, SF6 has 23900 and NF3 has 8000 which values are extremely high. Therefore, the compounds have a high greenhouse effect and affect global warming very greatly.
Further, in the case of using the perfluoro compounds such as CF4, C2F6, SF6, NF3 etc. as a plasma CVD chamber cleaning gas, exhaust gases include the perfluoro compounds themselves which compounds are not decomposed even if plasma is generated. Further, in the case of using various compounds containing carbon and fluorine such as perfluoro carbon, hydrofluoro carbon (HFC) etc., exhaust gases contain CF4, etc. which are generated as a by-product. So they induce an environmental problem of exhausting global warming gases.
The gases inviting the greenhouse effect and global warming gas contained in the exhaust gases as described above are hardly absorbed or decomposed by abatement apparatus (water scrubber) which apparatus has conventionally been used mainly, so that a decomposition apparatus by chemical reactions including burning with high temperature and high energy is required.
Further, in the case that these cleaning gases are used for plasma CVD chambers cleaning, a method that the cleaning gases are generated by plasma for cleaning has been attempted for attaining effective cleaning performance.
Usually, in plasma cleaning, a proper amount of an additional gas such as oxygen, argon etc. is mixed with the cleaning gases.
According to the knowledge of the present inventors, in the mixed gas system of the cleaning gases and the additional gases, with increasing the concentrations of the cleaning gases in conditions that the total gas flow rates are constant, the etching rates tend to be increased. However, when the cleaning gas concentrations are over constant concentrations, there are problems such as instability in plasma generation, slowdown or lowering of the etching rate, deterioration in uniformity of cleaning etc. Particularly, using the cleaning gas in a concentration of 100% induces a more marked tendency toward instability in plasma generation, slowdown or lowering of the etching rate and deterioration in the uniformity of cleaning so that it has a problem such that it is lacking in practical use.
On this account, prior to use, the cleaning gases need to be diluted so as to have the peak concentration in the etching rate-cleaning gas concentration curve or a lower concentration than the peak concentration. In order to depress the lowering of the etching rate caused with the dilution, the cleaning condition has been optimized by increasing the chamber pressure or increasing the gas flow rate in cleaning. However, when the chamber pressure is enhanced or the gas flow rate is increased in cleaning, the plasma generation becomes unstable, so that the uniformity of cleaning is deteriorated and efficient cleaning cannot be conducted.
Of the above cleaning gases, for example, in the use of NF3, a remote plasma apparatus in which plasma is generated other than a process chamber of a CVD apparatus is required for securing a high decomposition rate which is one of the objects so that the cleaning efficiency is improved, contamination of undecomposed NF3 into exhaust gases is avoided. However, the CVD apparatus provided with the remote plasma apparatus has a problem in that the apparatus is complicated and thereby the production cost is high.
Further, in conventionally used cleaning gases, when the chamber pressure is enhanced or the gas flow rate is increased, the decomposition rate of the cleaning gases in plasma and the plasma stability (uniformity of cleaning) are remarkably decreased so that the cleaning gases themselves are contained in the exhaust gases resulting in increasing the emission amount of global warming substances. Therefore, it is necessary to take measures for abatement of the gases.
Accordingly, plasma cleaning gases which are used for chamber cleaning after the CVD process in producing semiconductors such as LSI, etc., and have a low greenhouse effect and a low global warming potential, excellent uniformity of cleaning even in diluting conditions or not, and also have an effective etching rate, and a process for plasma cleaning has been desired.
In the light of the foregoing, the present inventors have earnestly studied for solving the above problems, and found that when a mixed gas of fluorine gas and a gas incapable of substantially reacting with fluorine in plasma is used, plasma can be generated stably without a remote plasma apparatus in wider conditions than the use of, for example, NF3 and further the excellent etching rate can be provided without lowering of the etching rate. The present inventors further found that for example, as will be described in Examples 34 and 37, and Comparative Examples 30 and 31 of the present specification, in the case that the total gas flow rate is 1000 sccm, when the chamber pressure condition is changed from 250 Pa to a higher pressure of 400 Pa, in the use of a mixed gas containing 20% by volume of NF3, the uniformity of cleaning is decreased, but in the use of a mixed gas containing the same 20% by volume of fluorine gas, the proper uniformity of cleaning can be secured.
Further, the present inventors found that when 100% by volume of fluorine gas is generated by plasma and then is used as a cleaning gas, the extremely excellent etching rate can be attained without the use of a remote plasma apparatus, and also even in conditions that the total gas flow rate is about 1000 sccm and the chamber pressure is about 400 Pa, plasma can stably be generated and the proper uniformity of cleaning can be secured.
Thus, the present invention has been accomplished.
The present invention is intended to solve the problems associated with the prior arts as described above, it is an object of the present invention to provide a plasma cleaning gas for CVD chambers which gas has excellent uniformity of cleaning and an effective etching rate in cleaning in the conditions that the total gas flow rate is about 1000 sccm and the chamber pressure is about 400 Pa under the dilution condition or under no dilution condition. It is a further object of the invention to provide a process for plasma cleaning CVD chambers.